The present invention relates to a beam splitter and an optical coupler which are used in an optical communication system and, more particularly, to a beam splitter which transmits about 85% of a beam incident to the beam splitter at about 45.degree. and reflects about 15% of the beam independently of the polarization state of the beam and an optical coupler using the same.
In recent years, demands for transmission of a large volume of information have been enhanced, an optical communication system using optical fibers has been used in place of a conventional communication system using copper cables. In order to constitute such an optical communication system, in addition to an optical active device for performing electrooptical conversion or photoelectric conversion, an optical passive device such as an optical coupler for branching an optical signal or an optical switch for performing time or spatial switching of optical paths is required.
Of the above devices, the optical coupler having a characteristic feature in which a beam can be branched at a predetermined ratio independently of the polarization state of an incident beam is desired. This characteristic feature is particularly required when the optical coupler is used in a system such as a single-mode optical fiber transmission system in which the polarization state of a beam traveling in an optical fiber changes.
According to a conventional technique, a beam splitter must be combined to a total reflection element to realize such an optical branching operation independent of polarization.
FIG. 4 shows a conventional optical coupler. Note that this optical coupler is proposed in Japanese Patent Laid-Open No. 62-269106. The optical coupler is constituted by optical fibers 31a to 31d, lenses 32a to 32d, total reflection elements 33a to 33d, a beam splitter 34 using a dielectric multilayer film, and a housing 35 for fixing these parts to each other. The total reflection elements 33a to 33d are arranged mainly to decrease the angle of incidence of a beam incident to the beam splitter 34 (close to normal incidence). For example, a beam guided inside the housing 35 by the optical fiber 31a and the lens 32a is totally reflected by the total reflection element 33a and then incident to the beam splitter 34 at a small angle of incidence. The beam incident to the beam splitter 34 is partially reflected, and the reflected beam is totally reflected by the total reflection element 33b and then output to the optical fiber 31b. The beam transmitted through the beam splitter 34 is guided to the optical fiber 31c by the total reflection element 33c.
Note that when a beam is incident to a conventional splitter at a large angle of incidence (e.g., 45.degree.), the branching ratio of the beam changes depending on the polarization state of the incident beam. In order to prevent this, the angle of incidence of a beam incident to the beam splitter 34 is decreased using the total reflection elements 33a to 33d.
As described above, in a conventional beam splitter, the angle of incidence of a beam must be suppressed to prevent a branching ratio from being dependent on the polarization state of the beam. For this reason, in order to constitute an optical coupler from which a beam can be output at a proper angle, the beam splitter must be combined to total reflection elements. Therefore, the optical coupler cannot be easily decreased in size.